1. Field of the Invention
The present invention relates to a sound image localization apparatus, and is suitably applicable to a sound image localization apparatus for localizing a sound image reproduced by a headphone to an optimal position.
2. Description of the Related Art
Multi-channel audio signals are abundantly used as the sound along with the picture such as a movie. It is presumed that such multi-channel audio signals to be recorded are regenerated with the speaker arranged to both sides of the graphic display plane such as a screen and in the center, and the speaker put on the back of the listener or both sides. A sound field to have a natural broadening for the sound image position of regenerated sound actually heard to be like the position of a sound source in the picture can be established by regenerating those audio signals using a set of speakers arranged to such fixed positions.
However, when such an audio signal is reproduced on a headphone apparatus, the sound image of the regenerated sound is localized in the head of the listener. Because of this, the position of a sound image of the regenerated sound does not align with the position of a sound source in the picture, giving rise to a very unnatural sound field. Also, the position of localization of the audio signal of each channel can not regenerate separately and independently, and therefore more than one musical sound like an orchestra is localized uniformly in the head to compose an unnatural sound field.
To improve unnatural localization of the sound image in such headphone apparatus, a headphone apparatus was proposed in which an impulse response from an optimal position of a speaker to both ears of the listener is measured or calculated, an impulse response concerned is convolved in the audio signal using the digital filter, and the audio signal is regenerated, thereby attaining auditory localization of the natural sound image which just regenerates from the actual speaker (e.g., refer to Japanese Patent Application Laid-Open No. 2000-227350).
FIG. 1 shows the configuration of a headphone apparatus 100 for auditorily localizing the sound image of audio signal on one channel. The headphone apparatus 100 converts an analog audio signal SA on one channel inputted via an input terminal 1 into digital form in an analog digital conversion circuit 2 to generate a digital audio signal SD, and supply it to the digital processing circuits 3L and 3R. The digital processing circuits 3L and 3R perform the signal processings of auditory localization for the digital audio signal SD.
When a sound source SP to be localized is in front of the listener M, as shown in FIG. 2, the sound outputted from the sound source SP arrives via a path having the transfer functions HL and HR to the left and right ears of the listener M. The impulse responses on the left and right channels in which the transfer functions HL and HR are transformed into the time axis are measured or calculated in advance.
The digital processing circuits 3L and 3R convolve the impulse responses on the left and right channels into the digital audio signal SD and output the digital audio signals SDL and SDR. In this connection, each of the digital processing circuits 3L and 3R is made up of a Finite Impulse Response (FIR) filter, as shown in FIG. 3.
The digital analog conversion circuits 4L and 4R convert the digital audio signals SDL and SDR into analog form to generate the analog audio signals SAL and SAR, which are amplified in the corresponding amplifiers 5L and 5R, and supplied to a headphone 6. And the acoustic units (electro-acoustic transducer elements) 6L and 6R of the headphone 6 convert the analog audio signals SAL and SAR into sound and output it.
Accordingly, the left and right reproduced sounds outputted from the headphone 6 become equivalent to the sounds arriving from the sound source SP via the path having the transfer functions HL and HR, as shown in FIG. 2. Thereby, when the listener wears the headphone 6 and listens to the reproduced sound, the sound image is localized at the position of the sound source SP as shown in FIG. 2 (i.e., auditory localization).
Referring to FIG. 4, a headphone apparatus 101 for localizing the sound image of a multi-channel audio signal out of the head will be described below. In this headphone apparatus 101, the audio signals on three channels are localized out of the head to the positions corresponding to the sound sources SPa, SPb and SPc, as shown in FIG. 5. The impulse responses in which the transfer functions HaL and HaR from a sound source SPa to both ears of the listener M, the transfer functions HbL and HbR from a sound source SPb to both ears of the listener M, and the transfer functions HcL and HcR from a sound source SPc to both ears of the listener M are transformed into the time axis are measured or calculated in advance.
In FIG. 4, an analog digital conversion circuit 2a of the headphone apparatus 101 converts an analog audio signal SAa inputted via an input terminal 1a into digital form to generate a digital audio signal SDa, which is supplied to the digital processing circuits 3aL and 3aR at the latter stage. Likewise, an analog digital conversion circuit 2b converts an analog audio signal SAb inputted via an input terminal 1b into digital form to generate a digital audio signal SDb, which is supplied to the digital processing circuits 3bL and 3bR at the latter stage. Also, an analog digital conversion circuit 2c converts an analog audio signal SAc inputted via an input terminal 1c into digital form to generate a digital audio signal SDc, which is supplied to the digital processing circuits 3cL and 3cR at the latter stage.
The digital processing circuits 3aL, 3bL and 3cL convolve an impulse response for the left ear into the digital audio signals SDa, SDb and SDc, and supply the digital audio signals SDaL, SDbL and SDcL to an addition circuit 7L. Likewise, the digital processing circuits 3aR, 3bR and 3cR convolve an impulse response for the right ear into the digital audio signals SDa, SDb and SDc, and supply the digital audio signals SDaR, SDbR and SDcR to an addition circuit 7R. Each of the digital processing circuits 3aL and 3aR, 3bL and 3bR, 3cL and 3cR is made up of the same FIR filter as the digital processing circuits 3L and 3R, as shown in FIG. 1.
The addition circuit 7L adds the digital audio signals SDaL, SDbL and SDcL, into which the impulse response is convolved, to generate a digital audio signal SDL on the left channel. Likewise, the addition circuit 7R adds the digital audio signals SDaR, SDbR and SDcR, into which the impulse response is convolved, to generate a digital audio signal SDR on the right channel.
The digital analog conversion circuits 4L and 4R convert the digital audio signals SDL and SDR into analog form to generate the analog audio signals SAL and SAR, which are amplified by the corresponding amplifiers 5L and 5R, and supplied to the headphone 6. And the acoustic units 6L and 6R of the headphone 6 convert the analog audio signals SAL and SAR into sound and output it.
At this time, the left and right reproduced sounds outputted from the headphone 6 become equivalent to the sounds arriving from the sound sources SPa, SPb and SPc via the paths having the transfer functions HaL and HaR, HbL and HbR, HcL and HcR, as shown in FIG. 5. Thereby, when the listener wears the headphone 6 and listens to the reproduced sounds, the sound images are localized at the positions of the sound sources SPa, SPb and SPc, as shown in FIG. 5. When the audio signals on four or more channels are dealt with, the sound image is auditorily localized in same way.
On the other hand, when the multi-channel audio signal is regenerated on the speakers, there is a problem that a number of speakers corresponding to channels may not be arranged due to the limited area of a listening room. According to an embodiment, there is an attempt for composing a number of sound images around the listener, employing a limited number of speakers.
FIG. 6 shows a speaker apparatus 200 for localizing the sound image at any position, employing two speakers 9L and 9R, in which an analog audio signal SA inputted via an input terminal 1 is converted into digital form by an analog digital conversion circuit 2 to generate a digital audio signal SD which is supplied to the digital processing circuits 8L and 8R.
The digital processing circuits 8L and 8R convolve an impulse response (hereinafter described) for localizing the sound image into the digital audio signal SD and output the digital audio signals SDL and SDR. Each of the digital processing circuits 8L and 8R is made up of the same FIR filter as the digital processing circuits 3L and 3R as shown in FIG. 1.
The digital analog conversion circuits 4L and 4R convert the digital audio signals SDL and SDR into analog form to generate the analog audio signals SAL and SAR, which are amplified by the corresponding amplifiers 5L and 5R, and supplied to the speakers 9L and 9R. And the speakers 9L and 9R convert the analog audio signals SAL and SAR into sound and output it.
The concept of a sound image localization process in the digital processing circuits 8L and 8R will be described below. A case where the sound sources SPL and SPR are disposed left and right forward of the listener M, and a virtual sound source SPx is equivalently revived (localized) at any position by the sound sources SPL and SPR will be considered, as shown in FIG. 7.
Herein, supposing the transfer functions
HLL: transfer function from sound source SPL to the left ear of the listener M
HLR: transfer function from sound source SPL to the right ear of the listener M
HRL: transfer function from sound source SPR to the left ear of the listener M
HRR: transfer function from sound source SPR to the right ear of the listener M
HXL: transfer function from virtual sound source SPX to the left ear of the listener M
HXR: transfer function from virtual sound source SPX to the right ear of the listener M
the sound sources SPL and SPR are given by the following expression.SPL=(HXL×HRR−HXR×HRL)/(HLL×HRR−HLR×HRL)×SPX  (1)SPR=(HXR×HLL−HXL×HLR)/(HLL×HRR−HLR×HRL)×SPX  (2)
Accordingly, the digital processing circuits 8L and 8R convolve an impulse response in which the transfer functions as in the expression (1) or (2) are transformed into the time axis into the digital audio signal SD to localize the sound image at the position of the virtual sound source SPx.
Though in the above description, the sound of audio signal on one channel is localized at any position by two speakers 9L and 9R, the sound of each of multi-channel audio signals may be localized at any position by two speakers, employing the same configuration as the multi-channel headphone apparatus 101, as shown in FIG. 4.
In the above headphone apparatus or speaker apparatus, the sound image is localized at any position by convoluting an impulse response based on the transfer function into the audio signal. However, when each of multi-channel audio signals is regenerated as the sound image having a clear spatial localization at any position, it may be required to convolve the impulse response having a sufficient length for each sound source, causing a problem that the digital processing circuit has an enormous amount of operation, making the configuration of the apparatus complex.
Therefore, there has been a need for a sound image localization apparatus which realizes localization of the sound image with a significantly reduced amount of operation.